When aggregate is transported from a mixing plant to a job site it is continuously mixed so that it will remain in a homogeneous unset condition. The mixing occurs within a container referred to as a drum or a bowl by those skilled in the art, the bowl being mounted for rotation on the bed of a self propelled truck. In order to obtain the correct physical properties of aggregate at the point of loading and at the point of discharge, it is necessary to measure the "slump". As previously stated, the aggregate is continuously measured both during the time it is charged and at the time of on-site discharge, i.e., during counterrotation of the container. The "slump" value is a reliable measure of the "set" of the aggregate, its pourability, and spreadability. The slump should be of a certain prescribed value at the time it is poured in order that it will completely fill the form without internal voids and have the desired physical characteristics of pourability, spreadability, etc. at the time of pour. Slump is measured at the time of the pour. During the pour, the drum is counterrotated from the direction in which it normally turns during transport. The slump is continuously measured during both rotation of the drum and counterrotation of the drum. The measurement is continuous in either direction. The direction of rotation of the drum does not have an effect on the measurement of the slump value. If the slump is not of the prescribed value at the building site, it must be adjusted by adding one or a combination of water, cement, gravel, sand. Generally, however, the slump is adjusted by adding water to the mix so that it does have the desired slump.
It is objectionable to make extensive adjustments of the slump at the building site because of delay in time which is expensive both to the transporter of the aggregate and the on site workers who cannot continue with the pour until the slump is the proper value.
It is therefore of substantial importance not only to know the slump at the time the aggregate is charged to the container but also to continuously monitor the slump during the transit time while the bowl is rotating and the internal aggregate is being mixed by the bowl rotation. If the slump does change between the time of charge and the time of pour the driver can determine that a slump change has occurred and what must be done to the aggregate in order to make it suitably adjusted. A tank of water carried on the vehicle and valve controlled by the operator of the vehicle is used to add water to the aggregate to adjust the slump during transit so that the proper slump is attained and an immediate pour can be performed by the operator. In this way, the delay time at the pour site is reduced, making faster deliveries and pours and reducing the down time of the construction workers. The difficulty, however, lies in the different qualities of aggregate and the different quantities of aggregate within the bowl. A slump readout should not be effected by the quantity of charge within the bowl; the slump value of the aggregate charge should be read independently of the amount of aggregate charge. The bowl does not always receive the same charge so that in an 11 yard capacity drum, the slump should read the same value as would the same aggregate in a 5 or 4 or 3 yard charge. It is necessary, therefore, to have a slump readout device which monitors the aggregate regardless of the amount of charge within the bowl. Also, since the mix ratio changes it is necessary to have a slump readout which will provide the necessary slump monitoring regardless of the mix ratio.
In order therefore for an accurate and practical slump monitoring device to be operative, it must be independent of the amount of charge and also it must adequately provide accurate readouts for different mix ratios which are called for. As will be seen in considering the objects of the invention both of these factors are adequately provided for in the present invention.